An image forming apparatus which includes a main body which has a printer engine which forms an image on a recording medium, a supply unit which supplies the recording medium to the main body, and a discharge unit which receives the recording medium, on which an image is recorded, discharged from the main body. The supply unit and discharge unit detachably connect to the apparatus and the units are stackable under the main body, and the supply unit and discharge unit have a relaying device for relaying the recording medium which is supplied or discharged from another unit or the main body.